I have a while True loop that needs to be run in the background to update a variable and a function that needs to return that same variable. I tried this as a test:
import threading
downspeed = 0
def updating():
while True:
downspeed = downspeed+1
def main():
while True:
print(downspeed)
u = threading.Thread(name='background', target=updating)
m = threading.Thread(name='foreground', target=main)
u.start()
m.start()
But it only returns 0
Your code is not really a sufficient test. The test ought to include:
See this code:
import threading
import time
can_run = True
downspeed = 0
def updating():
global downspeed
while can_run:
downspeed = downspeed+1
time.sleep(0.1) # Allow taskswitches
def main():
while can_run:
print(downspeed)
time.sleep(1) # Allow taskswitches
u = threading.Thread(name='background', target=updating)
m = threading.Thread(name='foreground', target=main)
u.start()
m.start()
time.sleep(10) # Allow time for the threads to do what they want
can_run = False
print('Done')
u.join()
m.join()
There are no problems sharing variables in python because of the GIL. Although this only makes changes atomic.
global downspeedat the beginning of the function definition ofupdating(inside the function). formain, sincedownspeedis not defined in the function scope, it will use the global scope, but if you set a variable within a function, it will become local unless you useglobal(and in this caseupdatingshould error, I believe)